A device to be charged such as a smart phone has become an essential tool for a customer in daily work and life. However, the device to be charged has disadvantages of low charging efficiency and long charging time.
When a set of battery cells coupled in series in the device to be charged are charged, it is required to ensure the battery cells are charged equally, otherwise performance and service life of the battery cell are affected during the usage. However, the device to be charged has disadvantage of low efficiency in voltage equalization.